In some engines, an electronically controlled throttle is used for improved performance. In addition, engines also may be controlled using engine output, or torque control where the actual engine torque is controlled to a desired engine torque through an output adjusting device, such as with the electronic throttle, ignition timing, air-fuel ratio, or various other devices.
In the electronic throttle control system, it is desirable to incorporate various features, such as cruise control and traction control. On approach for providing this type of integrated control system is described in U.S. Pat. No. 5,400,865. In this system, a desired torque is calculated that provides constant vehicle speed running. Also, a desired torque is calculated that prevents wheel slippage. Finally, a desired torque is calculated that is requested by the operator. A maximum of the driver requested torque and cruise control torque is first selected, and then the minimum of this result and the traction control torque is selected as the final desired torque. Then, the vehicle's powertrain is controlled to provide the finally selected torque.
The inventors herein have recognized a disadvantage of the above approach. In particular, with prior art torque arbitration schemes, the maximum of driver demanded torque and cruise control torque was selected. However, if negative torque is needed to control vehicle speed during cruise operation, such as down a hill, such control is not available since zero torque is typically the minimum torque allowed when the driver is not actuating the pedal. Alternatively, If negative torques are used in the driver demand table, then tip-out performance, or foot off pedal performance, and coasting driveability are sacrificed on level roads. Driveability is Oacrificed in level roads since the negative torque requests may cause downshifts on some tip-outs. Such transmission shifting can degrade drive feel on level roads.